1. Field of the Invention
The present invention relates to a vehicle brake control system. More particularly, the present invention relates to a vehicle brake control system having brake control means for controlling the operation of a regular brake device, based on a following distance from a preceding vehicle.
2. The Prior Art
There is a known ACC (Adaptive Cruise Control) system that carries out constant speed travel control, for maintaining a set vehicle speed when there is no preceding vehicle. The known ACC system also provides constant following distance control for maintaining a set following distance from a preceding vehicle, when there is a preceding vehicle. In the known ACC system, during constant speed travel control, the vehicle is automatically accelerated when the actual vehicle speed, detected by vehicle speed detection means, becomes less than the set vehicle speed. Similarly, during constant following distance control, the vehicle is automatically accelerated by the known ACC system when the actual following distance from a preceding vehicle, detected by preceding vehicle detection means, such as a radar or camera, exceeds the set following distance. Conversely, when the actual vehicle speed exceeds the set vehicle speed, or when the actual following distance becomes less than the set following distance, the vehicle is automatically decelerated.
The following paragraph describes operation of a system given a congestion tracking function, in which the application region of the known ACC system is extended to a lower speed region during congestion, when a preceding vehicle is repeatedly stopping and starting.
When the preceding vehicle detected by the preceding vehicle detection means stops, using the known ACC system, the subject vehicle is automatically stopped to keep pace with the preceding vehicle, and when the preceding vehicle starts, the subject vehicle is automatically started to keep pace with the preceding vehicle.
The known ACC system, having the above-mentioned congestion tracking function, conventionally employs the following automatic braking means in order to automatically generate a braking force without a braking operation by a driver when the vehicle is stopped.                1. The degree of communication between a vacuum booster and the atmosphere is controlled by a solenoid valve, so as to increase the difference between atmospheric pressure and the vacuum booster pressure, thereby generating a brake fluid pressure in a master cylinder and supplying it to a brake cylinder.        2. A brake fluid pressure generated by an electric hydraulic pump is supplied to a brake cylinder via a solenoid valve.        3. A brake fluid pressure generated by an electric hydraulic pump is stored under pressure in an accumulator, and the brake fluid pressure is supplied to a brake cylinder via a solenoid valve.        
However, the known braking means 1 to 3 require the solenoid valve and the electric hydraulic pump to be energized, not only when generating a brake fluid pressure, but also when maintaining the brake fluid pressure. This requirement of continuously energizing the solenoid valve and hydraulic pump can lead to problems of increased power consumption and heat generation in the solenoid valve, the electric hydraulic pump, and their controllers, thus degrading their durability.
If cooling means such as an electric heat-dissipating fan is provided in order to solve the problem of heat generation, there are provided new problems of an increase in cost, an increase in dimensions, and a further increase in the power consumption.
The present invention has been achieved in view of the above-mentioned circumstances, and it is an object of the present invention to save power while a vehicle is maintained in a stationary state, after the vehicle has been stopped by a regular brake device.